Windows Memcache Compatibility

Fixes an incompatibility issue while running MPOS on a Windows server. The PHP extension for using memcached on *nix systems is `Memcached`, and the extension is `Memcache` on Windows systems. In addition to the difference in class name on operating systems, the `set()` method has a different signature between the two classes. 

This patch adds a small wrapper class which provides compatibility between the two classes, so MPOS can run on Windows without any problems.

There are two alternatives to this patch.

* Disable memcache when running MPOS on Windows.
* Don't run MPOS on Windows.

The incompatibility between memcached extensions is the only thing I've found that keeps MPOS from running flawlessly on Windows.